Controllers Revamp Isolated DC-DC Converter Designs

The task of designing an isolated dc-dc converter for mainstream applications has been simplified in recent months by several advanced PWM controllers. The new controllers mainly target telecom dc-dc applications, where wide (36-V to 72-V) input-voltage range is a common requirement. However, many of these ICs can also find use in other dc-dc converter designs, as well as in off-line ac-dc supplies.

These controllers implement a range of popular converter architectures; support current-mode control in most cases; and bring new functions, such as high-voltage start-up circuitry, on chip. With some of the new chips, fixed and programmable functions also have been integrated, eliminating some external components.

Some of these components pursue integration primarily for cost or component reduction, while others as a way to make isolated converter designs accessible to the non-power design expert. Nevertheless, many features found in the new controllers provide experienced designers with greater flexibility in making performance trade-offs.

Synchronous Rectification/Programmability

One device due out shortly from Linear Technology is the LTC3723 synchronous push-pull PWM controller. This chip supports a variety of converter architectures, including forward, half bridge, full bridge and push-pull, mainly targeting applications such as quarter- and half-bricks in the 100-W to 750-W output range.

The LTC3723 is distinguished by its ability to support synchronous secondary rectification for high efficiency, while also providing key timing adjustments. For example, users can adjust the dead time on the primary-side switch and turnoff delays on the synchronous rectifiers. There's also adjustable undervoltage lockout and hysteresis, adjustable leading-edge blanking and adjustable slope compensation. In a typical application, the LTC3723 converts a 36-V to 72-V input to 12 V at 15 A (Fig. 1).

The controller provides a choice of current-mode control (LTC-3723-1) or voltage-mode control (LTC-3723-2). Also, the CMOS devices draw 3 mA of operating current and 145 µA of start-up current. Offered in 16-pin SSOPs, the chips are priced from $4.55 in 1000-piece quantities.

The MAX5068 current-mode controller recently unveiled by Maxim Integrated Products can be used to create a high-performance PWM converter in any popular topology. The controller allows the switching frequency to be set via an external resistor at frequencies up to 1.25 MHz. The switching frequency is trimmed internally for ±4% accuracy overtemperature. In contrast, many existing PWM controllers specify ±10% accuracy, which is not necessarily a guaranteed maximum overtemperature. The tighter tolerance on the frequency setting helps designers avoid oversizing their magnetics.

Maxim's controller also features digital soft-start, which ensures the output ramps up monotonically and avoids overshoot at power-up. With digital soft-start, the output turn-on time is based on several cycles for a given switching frequency. As a result, the turn-on time is said to be more predictable than with analog soft-start techniques that rely on the charging of an external capacitor. Other key features include an on-chip voltage reference with better than 1% accuracy, adjustable slope compensation and leading-edge blanking. In addition, the controller's gate-driver output is capable of sourcing or sinking 1 A.

Start-up current is just 50 µA, allowing use of a high-value start-up resistor, which reduces power losses in standby mode. Available in a thermally enhanced 16-pin TSSOP, the MAX5068 is priced starting at $1.22 in 1000-piece quantities (Fig. 2).

Replacing Mag Amps

Although a number of controllers exist for converting 48 V to a single low-voltage output in a single stage of conversion, the choices typically are more limited when multiple outputs are required. Magnetic amplifiers have been used for this purpose but require one transformer winding per output. Semtech has developed a lower-cost alternative to the mag amp with its SC4901 secondary-side controller. Using the company's Combi-Sync architecture, the controller performs both synchronous rectification and post regulation.

Unlike other approaches, the controller provides independent regulation of multiple outputs from a single transformer winding, allowing use of less-expensive magnetics. Individual outputs can be turned on or off without affecting the other outputs. In addition, each output has its own soft-start, remote sense and current limits.

Multiphase Control and Active Clamp/Reset

Texas Instruments put another twist on dc-dc designs when it introduced its interleaved PWM controllers for isolated applications this past fall. The UCC28220 and UCC28221 peak current-mode controllers employ dual-phase operation to generate a single 50-A to 100-A output. Unlike multiphase controllers developed for nonisolated designs, these controllers allow greater than 50% duty-cycle operation on each output.

This characteristic allows controllers operating in a forward converter architecture to achieve cancellation of output ripple. Ripple currents are reduced by 40% versus that of a single-phase implementation or an interleaved converter where the two phases don't overlap. The multiphase approach also offers the other benefits associated with nonisolated implementations — smaller size for external passives and faster transient response.

In addition, the UCC28221 contains a 110-V start-up JFET that eliminates a trickle charge boost strap resistor. The chips also provide accurate under- and overvoltage sensing on the input line. This programmable feature allows the user to set lower-and upper-voltage limits with hysteresis on the input range with the addition of just a few external resistors. Packaging is in 16-pin SOPs for both parts, and in 16-pin or 20-pin TSSOPs (the larger one is for the UCC28221.) In quantities of 1000, the UCC28220 and UCC28221 are priced starting at $1.49 and $1.57, respectively.

Another recent development is National's LM5025, a PWM controller for building forward converters using the active clamp/reset technique. This chip, which can control either a p-channel or n-channel clamp switch, is designed to reduce parts count and simplify the overall converter design through its integration of a startup regulator and a 3-A compound main gate driver. The 100-V linear regulator on chip eliminates external start-up bias circuitry, while enabling the controller to operate over a wide input range of 13 V to 90 V.

All of these functions have been squeezed into a 5-mm × 5-mm thermally enhanced chipscale package, the 16-pin LLP. A TSSOP version also is offered. In quantities of 1000, the LM5025 is priced at $1.30. (For more about this device in the application, see “Active Clamp Resets Transformer in Converters,” Power Electronics Technology, January 2004)

New Twist on Familiar Chips

Several months ago, Micrel Semiconductor revamped two industry-standard push-pull controllers. Its MIC3808 and MIC3809 are essentially second-source versions of similarly numbered models. However, Micrel's chips offer leading-edge blanking, which reduces the external filtering required on the current-sense signal. Like the parts they replace, the MIC3808/3809 are offered in the familiar 8-pin SOPs. However, they're also available in the 40% smaller 8-pin MSOP.

Micrel has taken these controllers a step further by developing two unique variations, the MIC3838 and MIC3839, available in 10-pin MSOPs (Fig. 3). The two extra pins on these packages allow the voltage reference and oscillator ramp signal to be brought outside the chip. This gives designers the option of using the chips either in current-mode or voltage-mode control. Pricing for the controllers starts at $1.47 in quantities of 1000.